Installing a water softener

Your vision is likely not going to be a reality. These unit must be mounted vertically and they are not very small. If all you want is info as to where you should put the filters, the filters before the WS is best. From there you will need to address how, what, where the WS will be located and if your plan will actually allow you to use the hardware you have purchased (plan to purchase).
There's really no reason not to use a portable on the ground outside the coach (the most convenient and easiest to service) but if you want to reinvent the wheel you are going to have to do some serious cyphering!